A multi-carrier system has recently been in the spotlight. The multi-carrier system means a system which supports a broadband by collecting one or more Component Carriers (CCs), each having a bandwidth smaller than the broadband, that is, a target when a wireless communication system supports the broadband. That is, a plurality of component carriers can be used in a multi-carrier system. The component carrier is defined by a center frequency and a bandwidth.
A component carrier on which a base station sends a signal to a terminal is called a downlink component carrier, and a component carrier on which a terminal sends a signal to a base station is called an uplink component carrier. One uplink component carrier and one downlink component carrier correspond to one cell. Accordingly, it can be said that a terminal supplied with service using a plurality of downlink component carriers is supplied with the service from a plurality of serving cells.
In a multi-carrier system, the number of uplink control signals transmitted can be increased as compared with the existing single carrier system. For example, in a multi-carrier system, a terminal cannot receive a plurality of Transport Blocks (TBs) through a plurality of downlink component carriers. In this case, the number of uplink control signals can be increased as compared with a single carrier system because the terminal must send a Hybrid Automatic Repeat Request (HARQ) acknowledgement/non-acknowledgement (ACK/NACK), that is, an ACK/NACK response, to each of the transport blocks. Furthermore, in a multi-carrier system, there may be a limit that an uplink control signal has to be transmitted through one uplink component carrier. Accordingly, there is a need for a method that is different from a method of sending an uplink control signal in the existing single carrier system because increased uplink control signals may have to be transmitted through one uplink component carrier.
Furthermore, in a multi-carrier system, a transport block that can be transmitted through one downlink component carrier may differ according to a transmission mode. For example, only one transport block can be transmitted or a maximum of two transport blocks can be transmitted in a physical downlink shared channel (PDSCH) depending on transmission mode of a downlink component carrier. Furthermore, there may be a case where only one transport block is transmitted through a downlink component carrier set in transmission mode in which a maximum of two transport blocks can be transmitted. In this case, how a terminal will send HARQ ACK/NACK using what method may be problematic.